The present invention relates to a process for preparing modified polyolefin particles and a foamed article made from the particles.
A foamed article made of a polyolefin, especially polyethylene is suitable to a good packing or wrapping material, because the foamed article has excellent toughness, is not easy to be broken and can be repeatedly used. However, the foamed article has drawbacks such as low accuracy of dimension due to its great shrinkage at expansion-molding and low cushioning property due to its limpness. Therefore, there is difficulty in use as a packing and a wrapping material for heavy and large goods. Those drawbacks result from low rigidity of polyolefin which is one of the inherent properties of the polyolefin. In order to improve the rigidity and the cushioning property of a polyolefin foam, there is used a modified polyolefin prepared by impregnating a polyolefin with a polymerizable vinyl monomer, especially styrene, capable of dissolving the polyolefin, and polymerizing the vinyl monomer.
As a process for preparing modified polyolefin particles, there is proposed, for instance, a process in which modified polyolefin particles are prepared by dispersing polyolefin particles into water, adding thereto a styrene monomer which dissolves a catalyst capable of initiating polymerization of the styrene monomer and a cross-linking agent capable of cross-linking the polyolefin, impregnating the polyolefin particles with the styrene monomer, polymerizing the styrene monomer and cross-linking the polyolefin.
According to the above process, however, the polyolefin particles impregnated with the styrene monomer are quite easy to agglomerate with each other and the agglomerated particles are hardened to form a mass in the course of the polymerization of the styrene monomer. When the polymerization of the styrene monomer is carried out at a temperature so that the polyolefin particles are molten, the particles are far easy to agglomerate with each other because the particles tend to melt and fuse with each other. Therefore, the polymerization is generally carried out at a temperature of not more than the softening point of the polyolefin. In order to carry out the polymerization at that condition, it is required that the vinyl monomer is gradually added over a long time for completing the uniform impregnation of the styrene monomer. According to the procedure, however, since the styrene monomer is hard to be introduced into the particles, the styrene monomer tends to be polymerized in water before the impregnation.
For improving the procedure in the impregnation of the vinyl monomer, there are proposed a process in which another particular polyolefin having low softening point is used and a process in which a softening agent such as dioctyl phthalate is added to the polyolefin.
In the above-mentioned processes, however, there is a drawback that a large amount of a cross-linking agent is required since the cross-linking agent is consumed in the polymerization of the styrene monomer.
There is also proposed another process for preparing modified polyolefin particles by dissolving cross-linked polyolefin particles in a large amount of an organic solvent, adding a methacrylate monomer thereto, and polymerizing the monomer. In the process, the impregnation of the methacrylate monomer is carried out at a temperature of less than the softening point of the polyolefin which is not yet cross-linked. According to that process, however, the particles are easy to stick and agglomerate with each other, and the impregnation cannot be carried out at a high temperature and for a short time, since the cross-liked polyolefin particles are dissolved in a large amount of the organic solvent. Furthermore the process requires much labor for removing the organic solvent used for dissolving the cross-linked polyolefin after completion of the polymerization.
Another process is proposed in Japanese Examined patent publication No. 46138/1976 for employing various polyolefins having various softening points. In the process, modified polyolefin particles are prepared by impregnating polyolefin particles with a styrene monomer, polymerizing the monomer, and then cross-linking the polyolefin. According to the process, however, the obtained modified particle merely contains large styrene polymer particles dispersed therein because the impregnation of the styrene monomer occurs only between the molecules in the amorphous portion of the polyolefin. As a result, cells in the foamed article obtained from such particles are large and are not uniform.
In general, modified polyolefin particle in which fine vinyl polymer particles are uniformly distributed can provide a foamed article having excellent cushioning property, accuracy of dimension, surface touch, and the like. The vinyl polymer particles in the modified polyolefin particles obtained by the conventional processes, however, vary in size and are not uniformly distributed, and also cells in the obtained foamed article are large and are not uniformly distributed, and the number of the cells is small, i.e. at most about 150 per 1 mm.sup.2 of the foamed article. Those foamed articles are easy to shrink, rough to the touch and inferior in cushioning property. Therefore, the conventional foamed articles are not suitable as a packing material for goods which need tough protection from any external impact, such as a camera, a watch and an electronic printer, especially for goods which are easy to be injured by the rough surface of the packing materials, such as a high-class ornamental article.
An object of the present invention is to provide a process for preparing modified polyolefin particles in which impregnation of a vinyl monomer can be rapidly carried out without any agglomeration of particles, the modified polyolefin particles which have fine polymer particles of the vinyl monomer uniformly distributed therein, and are usable in preparation of a foamed article.
Another object of the present invention is to provide a foamed article of polyolefin which includes uniform and fine cells and is suitable for a packing or wrapping material for goods to be protected from external impact and surface injury.
These and other objects of the present invention will become apparent from the description hereinafter.